Headwall Scarps Research Articles

Sequence of Multiple Slope Failures in the Headwall Area of the Giant Sahara Slide Complex at the NW African Continental Margin

AbstractSubmarine mega‐slides involving hundreds of cubic kilometers of slope material pose a major threat due to their potential to destroy offshore infrastructure and trigger devastating tsunamis. The Sahara Slide Complex affected about 50,000 km2 of the northwestern (NW) African continental margin. Previous studies focused either on its distal depositional zone or the uppermost headwall area, but failed in reconstructing the succession of individual slide events within the entire headwall area. New hydroacoustic data reveal a complex slide morphology including three main acoustic facies, large scale slide blocks, linear troughs, multiple glide planes and three major headwall scarps (the upper, southern and lower headwall). The evacuated slide scar hosts chaotic slide deposits that cover stratified sediments in the upper and southern headwall area, but are vertically stacked onto older slide deposits in the lower headwall area. Based on these observations, and dating of recently collected sediment samples, we reconstructed the history of slope failures that led to the formation of the structurally and morphologically complex headwall area of the Sahara Slide. Slope instability initiated when the lower headwall failed at ∼60 kyr, followed by the failure of the northeastern upper headwall at ∼14 kyr. Around 6 kyr, a major slide within the upper headwall area took place, followed by a series of smaller events within the southern and most‐proximal upper headwall area. The youngest of these slides occurred around 2 kyr. This scenario suggests a long‐lasting history of successive slope failures for the Sahara Slide Complex along the NW African continental slope.

The role of mass‐transport complexes in the initiation and evolution of submarine canyons

AbstractThe offshore area of the Otway Basin (south‐eastern Australia) is dominated by multibranched canyons where mass‐transport complexes are widely distributed. This study integrates high‐resolution multibeam and seismic data to investigate the importance of mass‐transport complexes in dictating the evolution of canyons. The study interprets three regionally distributed mass‐transport complexes that fail retrogressively and affect almost 70% of the study area. Within the mass‐transport complexes, seven canyons that initiated from the continental shelf edge and extended to the lower slope are observed. Although the canyons share common regional tectonics and oceanography, the scales, morphology and distribution are distinctly different. This is linked to the presence of failure‐related scarps that control the initiation and formation of the canyons. The retrogressive failure mechanisms of mass‐transport complexes have created a series of scarps on the continental shelf and slope. In the continental shelf, where terrestrial input is absent, the origin of the canyons is related to local failures and contour current activities, occurring near the pre‐existing larger headwall scarps (ca 120 m high, 3 km long). The occurrence of these local failures has provided the necessary sediment input for subsequent gravity‐driven, downslope sediment flows. In the continental slope, the widespread scarps can capture gravity flows initiated from the continental shelf, developing an area of flow convergence, which greatly widens and deepens the canyons. The gradual diversion and convergence through mass‐transport complex related scarps have facilitated the canyon confluence process, which has fundamentally changed the canyoning process. Thus, this study concludes that the retrogressive failure mechanism of mass‐transport complexes has a direct influence on the initiation, distribution and evolution of the canyons. The scarps associated with mass‐transport complexes have greatly facilitated the delivery of sediments and marine plastics from the shelf edge into the deep oceans, especially in areas where fluvial input is missing.

Geomorphometric descriptions of archipelagic aprons off the southern flanks of French Frigate Shoals and Necker Island edifices, Northwest Hawaiian Ridge

AbstractThis study describes the geomorphometries of archipelagic aprons on the southern flanks of the French Frigate Shoals and Necker Island edifices on the central Northwest Hawaiian Ridge that are hotspot volcanoes that have been dormant for 10–11 m.y. The archipelagic aprons are related to erosional headwall scarps and gullies on landslide surfaces but also include downslope gravitational features that include slides, debris avalanches, bedform fields, and outrunners. Some outrunners are located 85 km out onto the deep seafloor in water depths of 4900 m. The bedforms are interpreted to be the result of slow downslope sediment creep rather than products of turbidity currents. The archipelagic aprons appear to differ in origin from those off the Hawaiian Islands. The landslides off the Hawaiian Islands occurred because of oversteepening and loading during the constructive phase of the islands whereas the landslides off the French Frigate Shoals and Necker Island edifices may have resulted from vertical tectonics due to the uplift and relaxation of a peripheral bulge or isolated earthquakes long after the edifices passed beyond the hotspot. The lack of pelagic drape in water depths above the 4600 m depth of the local carbonate compensation depth suggests that the archipelagic apron off the French Frigate Shoals edifice is much younger, perhaps Quaternary in age, than that off the Necker Island edifice, which has a 50 m pelagic drape. The pelagic drape off the Necker Island edifice suggests that the landslides may be as old as 9 Ma. The lack of pelagic drape off the French Frigate Shoals edifice suggests that the most recent landslides are more recent, perhaps even Quaternary in age. The presence of a chute-like feature on the mid-flank of the French Frigate Shoals edifice appears to be the result of rejuvenated volcanism that occurred long after the initial volcanism ceased to build the edifice.

Lateral variability of shelf-edge and basin-floor deposits, Santos Basin, offshore Brazil

ABSTRACTConstruction of continental margins is driven by sediment transported across the shelf to the shelf edge, where it is reworked by wave, tide, and fluvial processes in deltas and flanking clastic shorelines. Stalling of continental-margin progradation often results in degradation of the outer shelf to upper slope, with resedimentation to the lower slope and basin floor via a range of sediment gravity flows and mass-movement processes. Typically, our understanding of how these processes contribute to the long-term development of continental margins has been limited to observations from broadly two-dimensional, subsurface and outcrop datasets. Consequently, the three-dimensional variability in process regime and margin evolution is poorly constrained and often underappreciated. We use a large (90 km by 30 km, parallel to depositional strike and dip, respectively) post-stack time-migrated 3D seismic-reflection dataset to investigate along-strike variations in shelf-margin progradation and outer-shelf to upper-slope collapse in the Santos Basin, offshore SE Brazil. Early Paleogene to Eocene progradation of the shelf margin is recorded by spectacularly imaged, SE-dipping clinoforms. Periodic failure of the outer shelf and upper slope formed ca. 30-km-wide (parallel to shelf-margin strike) slump scars, which resulted in a strongly scalloped upper-slope. Margin collapse caused: 1) the emplacement of slope-attached mass-transport complexes (MTCs) (up to ca. 375 m thick, 12+ km long, 20 km wide) on the proximal basin floor, and 2) accommodation creation on the outer shelf to upper slope. This newly formed accommodation was infilled by shelf-edge-delta clinoforms (up to 685 m thick), that nucleated and prograded basinward from the margin-collapse headwall scarp, downlapping onto the underlying slump scar and/or MTCs. Trajectory analysis of the shelf-edge deltas suggests that slope degradation-created accommodation was generated throughout the sea-level cycle, rather than during base-level fall as would be predicted by conventional sequence-stratigraphic models. Our results highlight the significant along-strike variability in depositional style, geometry, and evolution that can occur on this and other continental margins. Coeval strata, separated by only a few kilometers, display strikingly different stratigraphic architectures; this variability, which could be missed in 2D datasets, is not currently captured in conventional 2D sequence stratigraphic models.

Factors leading to slope failure on a sediment-starved margin: The southwestern continental margin of the East Sea, Korea

Submarine landslides are common geomorphological features of continental margins. Some of the largest submarine landslides occurred on low-angle (< 4°), sediment-starved margins, yet their preconditioning and trigger mechanisms are still largely unconstrained. The southwestern continental margin of the East Sea (between 37.5°N and 38.0°N), Korea, occupies a narrow shelf (< 10 km), is characterized by low sedimentation rates (~3–7 cm /ka) with an average gradient of less than 2°. Here, we investigate submarine landslides using newly collected datasets including multibeam echosounder (MBES), chirp sub-bottom profiler, multichannel seismic (MCS) data and ten piston cores. MBES data from the margin reveal at least four major submarine landslides initiated at depths of 400 m to over 600 m. These landslides left clear headwall scarps on the seafloor with reliefs reaching over ~130 m and extend for over 40 km. MCS data show that some of the failures have resulted in the complete disintegration of the failed mass, while others have resulted in the deposition of well-defined hummocky debris flows. Sediments recovered downslope of the headwall scarps contain slides and debris flow deposits and turbidites that are overlain by bioturbated hemipelagic layers. Radiocarbon dating from hemipelagic units overlying MTDs within the headwall scarps reveal that major failures occurred at ca. 11 to 19 ka, coinciding with the time of the Last Glacial Maximum (LGM) to early deglaciation. Since then, hemipelagic sedimentation has prevailed throughout the sediment starved slope. Slope stability analyses based on geotechnical properties of sediments indicate that all areas are stable under static, and even stable under loads derived from earthquakes in instrumental records, but there were probably earthquakes in pre-historical records (i.e., with a longer recurrence interval) of potentially significant larger magnitude. We suggest that the preferential occurrence of major failures adjacent to the major faults on the lower slope may ultimately be tectonic-controlled although other factors may have contributed as well. Our work shows that coarse-grained clastic sediments are abundant in the shallow subsurface and that these higher-permeability units, often identified as weak layers, would focus fluid flow and could act as slip planes for slope failure. Our data also indicate that tectonic steepening and gas charging are other key parameters for controlling instability in sediment-starved margins.

Fluid seepage associated with slope destabilization along the Zambezi margin (Mozambique)

Evidences for active fluid seepages have been discovered along the Zambezi continental slope (offshore Southern Mozambique). These seepages are mostly associated with pockmarks which are aligned along a trend parallel to the slope and running closely upstream of the headwall scarp of a wide zone of slope destabilization. Fluid seepages are interpreted as a potential trigger for the slope destabilization. Acoustic anomalies within the water column have been interpreted as related to moderate bubble seepages mostly located outside and only punctually inside the destabilization zone. Exploration with the SCAMPI towed camera system in the widest pockmark (diameter 200 m wide) has shown fluid seepages associated to authigenic carbonate crusts and possibly bacterial mats. These fluid seepages are also associated to the presence of chemiosynthetic organisms (Vesicomyidae and Thyasiridae bivalves, Siboglinidae tubeworms). The sampled gas in the sediment corresponds mainly to CH4 of microbial origin, generated by hydrogenotrophic methanogenesis from a substrate of organic origin, i.e. a conventional process of genesis of microbial gas in the marine domain. No evidence for thermogenic gas was detected. Another type of pockmarks has been observed within the core of the slope destabilization zone. Most of these pockmarks are inactive in terms of fluid seepage at present time and are associated to carbonate buildups forming chimney geometries. They probably correspond to diagenetic chimneys of former fluid migration pathways that have been exhumed during the mass sliding and the surrounding depression are related to recurrent activity of strong lateral slope currents which have scoured the sediments around. The spatial organization of the slope destabilization features is considered as representative of the temporal evolution of the landslide giving information about the dynamics of slope instability processes. This proposed evolution started by scattered seepages of formation water with dissolved gas. Then free gas seepages appeared notably in the upper part of the slope. This was followed by progressive shallow deformation in the sediments downslope of the main gas seepages. Finally, the whole slope was destabilized forming imbricated landslides exhuming locally former diagenetic chimneys.

The Geomorphology of Submarine Channel Systems of the Northern Line Islands Ridge, Central Equatorial Pacific Ocean

More than 844,000 km² of the northern Line Islands Ridge mapped with multibeam bathymetry and backscatter provide unprecedented views of the geomorphology of this isolated area in the central equatorial Pacific Ocean. A compilation of all available multibeam data in the area reveals six extensive submarine dendritic channel systems that encompass a combined drainage area that exceeds 60,000 km². The channel systems occur in a predominately carbonate environment and are the longest calciclastic submarine channel systems mapped in the oceans to date. The channel systems occur in a carbonate-dominated region well above the carbonate compensation depth and have developed into the surface of basins that are surrounded by small guyots and seamounts that make up a discontinuous rim around the summit of the northern Line Island Ridge. The channels have mostly straight or gently curved well-developed tributaries and main reaches. Although the Line Island Ridge has been dated at 86 to 68 Ma old, the channels occur on the surface and are not buried by any significant sediment accumulations. Levees are very rare along the channel banks and no bathymetric expression of submarine fans was found where the channels exit onto the adjacent abyssal basins. There is sparse evidence of landslide deposits throughout the ridge although the flanks of the guyots exhibit numerous headwall scarps. The presence of plunge pools below the northwest escarpment, together with well-defined channels meters to hundreds of meters deep relative to the surrounding seafloor, suggests the channels might be relatively recent (perhaps late Neogene or even younger) features developed long after the ridge subsided more than a kilometer below sea level.

Three-dimensional mapping and kinematic characterization of mass transport deposits along the outer Kumano Basin and Nankai accretionary wedge, southwest Japan

Three-dimensional (3D) seismic data from the southern Kumano Basin of southwest Japan image a nested series of moderately sized mass transport deposits (MTDs) that slid from a slope along the seaward side of the forearc basin. The deposits are dated to be approximately 0.3 to 0.9 Ma. These MTDs are likely linked to the movement along a prominent out-of-sequence thrust (OOST) fault, regionally steeper slopes that would have existed during deposition, and shifts in sedimentation over the past 0.9 Ma. The spatial resolution provided by the 3D seismic data permits the identification of kinematic characteristics and the internal geometries of the MTDs which total over 2.8 km3 in volume and cover more than 59 km2 of the seafloor at various stratigraphic levels. Each MTD is well imaged and exhibits various kinematic indicators while most of the basal glide planes and original headwall scarps above the deposits have been partially or fully eroded by subsequent MTDs. There are at least seven individual deposits that range in volume from 0.005 to 1.16 km3, in area from 0.2 to 21.8 km2; have runouts between 0.55 and 7.9 km, and generally translate downslope from the SE to NW. Basal, internal, and top surface kinematic indicators, such as grooves, thrust and fold systems, and pressure ridges, show that these MTDs originate from a prominent slide scar recognized in the high-resolution regional bathymetry. This, combined with a regionally shifting depocenter and faulting related to the earthquake cycle, points to regional tectonic activity as being the most likely failure trigger for these nested landslides.

Mass-wasting processes along the margins of the Ulleung Basin, East Sea: insights from multichannel seismic reflection and multibeam echosounder data

Abstract Submarine landslides represent a major, previously little recognized, geological hazard to the coastal communities. This study investigates the size, depth and degree of submarine landslides along the margins of the Ulleung Basin and examines how the shelf morphology and sediment supply affect the style and occurrence of slope failures. The slopes have experienced at least 38 episodes of submarine failures, which have left clear arcuate-shaped scarps that initiate at water depths of 150–1120 m. Individual landslides comprise volumes over the range 0.1–340 km 3 , cover 20–800 km 2 on the seafloor and have runout distances of up to 50 km from the source. The headwall scarps are observed as being in excess of 500 m high. The height of scarps in the southern margin is significantly larger than in the western margin. Moreover, the volume of mass-transport deposits in the southern margin is also much higher compared to those from the western margin. The occurrence of the broad shelf (30–150 km wide) and high sedimentation rates in the southern margin might have led to large-scale slope failures. In contrast, the narrow shelf (&lt;20 km) and low sedimentation rates in the western margin would only have promoted small-scale mass-wasting events.

Link between growth faulting and initiation of a mass transport deposit in the northern Taranaki Basin, New Zealand

AbstractThe Neogene section in the northern Taranaki Basin, offshore New Zealand, displays an interaction among prograding clinoforms, listric growth faults formed at the base of slope and mass transport deposits that fill the growth fault depocentres. This study focuses on one of these systems, the Karewa Fault and mass transport deposit (MTD), in order to understand the genetic relationship between the fault and the MTD in its hangingwall depocentre, i.e. did the MTD fill existing accommodation space? Did the MTD trigger growth fault displacement? Or is there some other relationship? Most mass transport deposits are elongate in the transport direction and exhibit a length:width aspect ratio of more than 1. However, the 90 km2 Karewa Fault MTD is at least three times wider than it is long, which is atypical for MTDs reported in the literature, where ~80% have a length:width ratio &gt;1. The transport direction of the MTD is to the WNW, as indicated by the location and internal structure of the compressional toe and the headwall scarp region of the Karewa Fault. The structural and sequence geometries on seismic reflection data indicate the MTD formed during the late stage of growth fault activity, and locally truncates the upper part of the Karewa Fault. The MTD is inferred to have originated by local destabilization of the sediment package overlying the Karewa Fault related to the escape of overpressured fluids along the fault. The resulting MTD was translated locally by only a few kilometres. This unusual cause for an MTD also resulted in its atypical length–width–thickness aspect ratios.

Gloria Knolls Slide: A prominent submarine landslide complex on the Great Barrier Reef margin of north-eastern Australia

We investigate the Gloria Knolls Slide (GKS) complex on the Great Barrier Reef margin of north-eastern Australia, the largest extant mixed carbonate-siliciclastic province in the world. Based on the most complete bathymetric and sub-bottom profile datasets available for the region, we describe the main surface and subsurface geomorphologic characteristics of this landslide complex. The GKS forms a 20km along-slope and 8km across-slope indentation in the margin, extending from 250 to 1350m depth, and involves a volume of 32km3 of sediment remobilized during three events. Three main seafloor terrains can be distinguished based on seafloor morphology: a source area, a proximal depositional area and a distal depositional area. The source area includes a main headwall scarp with a maximum height of 830m and a secondary scarp at 670m depth. The proximal depositional area is flat and smooth, and lacks debris exposed on the seafloor. The distal depositional area has a hummocky surface showing a distinctive cluster of eight knolls and over 70 small debris blocks. A dredge sample from the top of the largest knoll at a depth of 1170m reveals the presence of a cold-water coral community. In the sub-bottom profiles, the mass-transport deposits in the GKS are identified below the background sediment drape as partially confined, wedge-shaped bodies of mostly weak amplitude, transparent reflectors in the proximal depositional area; and more discontinuous and chaotic in the distal depositional area. The failed sediment slabs of the GKS were evacuated, transported and disintegrated downslope in three events following a sequential failure process spreading successively from the lower slope to the upper slope. The first event initiated at the lower slope at the depth of the secondary scarp, moved downslope and disintegrated over the basin floor leaving coherent blocks. The subsequent second and third events were responsible for the formation upslope of the main scarp in the GKS. The timing of emplacement of the first GKS event, constrained by radiometric age of fossil biota from the surface of the largest slide block, was at least before 302±19ka. The presence of alternating mixed carbonate and siliciclastic lithologies that build the slope might have played an important role as a preconditioning factor in this region. Preliminary estimations suggest that unusually large seismic events were the most likely triggering mechanism for the GKS. This work contributes to the understanding of large mass-movement deposits in mixed carbonate-siliciclastic margins and provides a useful morphologic characterization and evolutionary model for assessing its tsunamigenic potential with further numerical simulations. In addition, the discovery of a cold-water coral community on top of the largest knoll has implications for identifying similar landslide-origin cold-water coral communities on the GBR margin.

Geophysical evidence and inferred triggering factors of submarine landslides on the western continental margin of the Ulleung Basin, East Sea

Submarine landslides form very complex depositional and erosional features on the seafloor, and their dynamics and triggering processes are yet to be understood completely. Numerous studies are being undertaken both because of the scientific significance but also for their potential harm to seafloor infrastructure and coastal areas. This study investigates the styles and causes of landsliding along the western margin of the Ulleung Basin in the East Sea, based on multiple sparker, subbottom profiler, multibeam echosounder and sediment core datasets collected in 2015. The bathymetric analyses indicate that the southern slope of the Ulleung Basin has experienced at least seven submarine failures. These failures left clear arcuate-shaped scarps that initiated at water depths of ~600 m. The observed headwall scarps have heights that exceed 60 m and appear to be the result of retrogressive-type failures. Seismic reflection data clearly image the basal sliding surface that is characterized by a prominent high-amplitude reflector. Chaotic-to-transparent seismic facies occur immediately downslope of the headwall scarps; these represent ~20 m thick landslide deposits. Gravity cores taken from areas adjacent to the scars suggest that these slides are older than ca. 97 ka. Interpretation of the present data shows that faults appear to cut recent sediments upslope of scarps, and that the slope may still be in an active phase of failure. Seismic data also image various overpressurized gases and/or gas fluids, as evidenced by the occurrence of pockmarks and seismic chimneys in upslope or adjacent areas of the scarps. Hence, earthquakes associated with tectonic activity and development of fluid overpressure may have acted as the main conditioning factor for destabilizing the slope sediments. Geotechnical stability analyses indicate that the sampled slope sediments are exceptionally stable under present-day conditions, even under seismic loading. This finding points to additional forces such as excess pore pressure caused by gas fluids at the times of slide emplacement.

Mass transport deposits and processes in the north slope of the Xisha Trough, northern South China Sea

Triple mass-transport deposits (MTDs) with areas of 625, 494 and 902 km2, respectively, have been identified on the north slope of the Xisha Trough, northern South China Sea margin. Based on high-resolution seismic reflection data and multi-beam bathymetric data, the Quaternary MTDs are characterized by typical geometric shapes and internal structures. Results of slope analysis showed that they are developed in a steep slope ranging from 5° to 35°. The head wall scarps of the MTDs arrived to 50 km in length (from headwall to termination). Their inner structures include well developed basal shear surface, growth faults, stepping lateral scarps, erosion grooves, and frontal thrust deformation. From seismic images, the central deepwater channel system of the Xisha Trough has been filled by interbedded channel-levee deposits and thick MTDs. Therefore, we inferred that the MTDs in the deepwater channel system could be dominated by far-travelled slope failure deposits even though there are local collapses of the trough walls. And then, we drew the two-dimensional process model and threedimensional structure model diagram of the MTDs. Combined with the regional geological setting and previous studies, we discussed the trigger mechanisms of the triple MTDs.

Large Scarps and Massive Slide on the Briatico–Bivona Shelf, Calabria, Italy: Potential Link with the Destructive September 1905 Earthquake

Stanley, J.-D.; Bernasconi, M.P., and Nickerson G.A.J., 2014. Large scarps and massive slide on the Briatico–Bivona shelf, Calabria, Italy: potential link with the destructive September 1905 earthquake.A multibeam sonar (MBS) survey of the northern Capo Vaticano shelf between the towns of Briatico and Bivona, Calabria, Italy, has revealed a markedly broken margin offset by three large scarps: the upper scarp (S1) at approximate depths of 16–22 m, the middle scarp (S2) at ∼36–58 m, and the deeper scarp (S3) at ∼88–101 m. The configuration of S1 shows high relief to 33 m and a slope to 59°, which suggests a fault or tectonic–gravitative origin. Substantial volumes of sediment, mostly sand, are being rapidly transferred from the inner shelf toward and across the upper shelf break, and one could expect a smoothing and burial of the upper scarp, but 10 MBS profiles show the sharp, angular shape of that feature. Some of the mass gravitative deposits just below some headwall scarps that extend to the shelf-slope that break NE of Briatico also reveal very recent origins. Additionally, a massive slide block north of Bivona, ∼350–400 m in length, appears to have detached from the inner shelf and shifted onto the middle shelf. It is proposed that several of these seafloor features may, in some manner, be associated with the powerful and destructive September 1905 earthquake that affected a large sector of Calabria. Five potential epicenters on the SW margin of Calabria have previously been proposed for that event, and this MBS study area is in proximity to one of those. It is recommended that two localities in the survey area, one NE of Briatico and the other north of Bivona, be selected as test zones for further geological and geophysical analyses.

Geomorphometry of a submarine mass-transport complex and relationships with active faults in a rapidly uplifting margin (Gioia Basin, NE Sicily margin)

Abstract Along the NE Sicily margin, between Capo Milazzo and Capo Peloro, in the offshore Gioia Basin area facing the Castanea Ridge, a mass-transport complex (MTC) was first imaged by multibeam data acquired by the end of the 1990s. It was interpreted as resulting from the stacking of different mass-transport deposits due to repeated failures of the shelf edge and upper slope area and of the nearby Villafranca slope channel-levee wedge. New high resolution swath bathymetry and seismic data, acquired in 2009–2011, allowed the complete coverage of the headwall area of the 48 km3 Villafranca frontally-confined slide, which represents the bulk of the MTC, and that was previously not mapped. Seven smaller individual zones of instability that represent the source areas of distinct mass-transport deposits emplaced in the upper and lower slope, having surface areas between 10 km2 and 60 km2, were also mapped. The new data show that, although the Villafranca slide and the subsequent more recent failures were not sourced from the same headwall area, they are similarly associated to the activity of the faults that characterize the NE Sicily rapidly uplifting margin. The Villafranca slide has its headwall along a normal fault parallel to the coast, which is visible in the shelf at 80–85 m water depth offsetting the seafloor for 30 m. The slab slide and the debris flow lobes have their headwalls along a NNE–SSW oriented normal fault, which offsets the seafloor for 30 m and is the offshore prolongation of a regional structure on land. The largest debris flow lobe in the area was actually generated through flow transformation mechanisms from an initial slump mass. The main headwall scarps are rectilinear and exploit regional normal faults along which the principal and larger slope failures (slide and slumps) root, possibly during episodes of accelerated uplift, recognized by several authors in the area, both in Late Pleistocene and during the Holocene. Successive morphological modifications and flow transformations, resulting from smaller scale failures (debris flows), generate more complex headwalls. In addition, the MTC is bounded to the NE by a recent reverse fault which prolongs in a structural ridge (Acquarone Ridge) where evidence of seafloor fluid escapes (pockmarks) is present, implying that compressive and transpressive structures can be active in the area and play an underestimated role in the instability of the NE Sicily margin.

Comment on “Basement-controlled multiple slope collapses, Rockall Bank Slide Complex, NE Atlantic” by A. Georgiopoulou, P.M. Shannon, F. Sacchetti, P.D.W. Haughton, S. Benetti [Marine Geology 336 (2013) 198–214

Abstract Georgiopoulou et al. present a reassessment of the Rockall Bank Slide Complex (RBSC) NE Atlantic using reprocessed bathymetry data combined with seismic reflection data to propose a deeper linkage between underlying Mesozoic faults and the location of the headwall scarps of the slide complex. Although the faults have a key role to play in defining the shape of the slope I am unclear as to how these two seemingly independent phenomena are linked as cited in the paper. Fluid flow is also explored by the authors as a mechanism for making the slope prone to failure; however there is not enough evidence to suggest that this is the case as direct evidence of such flow is not present on this margin. An indirect dating method using an average growth rate model to estimate the age of a seabed mound which is assumed to be a carbonate mound is used; however the likelihood of the mound being a volcanic mound is not fully explored nor discounted. Whilst any study of the RBSC is to be welcomed I am not convinced about the evidence used to support the conclusions and feel that the findings should take into account existing knowledge from the region.

Morphological indicators of Late Quaternary co-seismic vertical displacements and catastrophic wave impact in Lesvos Island (NE Aegean Sea)

Along the NE Sicily margin, between Capo Milazzo and Capo Peloro, in the offshore Gioia Basin area facing the Castanea Ridge, a mass-transport complex (MTC) was first imaged by multibeam data acquired by the end of the 1990s. It was interpreted as resulting from the stacking of different mass-transport deposits due to repeated failures of the shelf edge and upper slope area and of the nearby Villafranca slope channel-levee wedge. New high resolution swath bathymetry and seismic data, acquired in 2009–2011, allowed the complete coverage of the headwall area of the 48 km3 Villafranca frontally-confined slide, which represents the bulk of the MTC, and that was previously not mapped. Seven smaller individual zones of instability that represent the source areas of distinct mass-transport deposits emplaced in the upper and lower slope, having surface areas between 10 km2 and 60 km2, were also mapped.The new data show that, although the Villafranca slide and the subsequent more recent failures were not sourced from the same headwall area, they are similarly associated to the activity of the faults that characterize the NE Sicily rapidly uplifting margin. The Villafranca slide has its headwall along a normal fault parallel to the coast, which is visible in the shelf at 80–85 m water depth offsetting the seafloor for 30 m. The slab slide and the debris flow lobes have their headwalls along a NNE–SSW oriented normal fault, which offsets the seafloor for 30 m and is the offshore prolongation of a regional structure on land. The largest debris flow lobe in the area was actually generated through flow transformation mechanisms from an initial slump mass.The main headwall scarps are rectilinear and exploit regional normal faults along which the principal and larger slope failures (slide and slumps) root, possibly during episodes of accelerated uplift, recognized by several authors in the area, both in Late Pleistocene and during the Holocene. Successive morphological modifications and flow transformations, resulting from smaller scale failures (debris flows), generate more complex headwalls.In addition, the MTC is bounded to the NE by a recent reverse fault which prolongs in a structural ridge (Acquarone Ridge) where evidence of seafloor fluid escapes (pockmarks) is present, implying that compressive and transpressive structures can be active in the area and play an underestimated role in the instability of the NE Sicily margin.

Structure and evolution of mass transport deposits in the South Caspian Basin, Azerbaijan

ABSTRACTThe Quaternary to late Pliocene sedimentary succession along the margin of the South Caspian Basin contains numerous kilometre‐scale submarine slope failures, which were sourced along the basin slope and from the inclined flanks of contemporaneous anticlines. This study uses three‐dimensional (3D) seismic reflection data to visualise the internal structure of 27 mass transport deposits and catalogues the syndepositional structures contained within them. These are used to interpret emplacement processes occurring during submarine slope failure. The deposits consist of three linked structural domains: extensional, translational and compressive, each containing characteristic structures. Novel features are present within the mass transport deposits: (1) a diverging retrogression of the headwall scarp; (2) the absence of a conventional headwall scarp around growth stratal pinch outs; (3) restraining bends in the lateral margin; (4) a downslope increase in the throw of thrust faults. The results of this study shed light on the deformation that occurred during submarine slope failure, and highlight an important geological process in the evolution of the South Caspian Basin margin.

Mass-transport deposits in confined wedge-top basins: surficial processes shaping the messinian orogenic wedge of Northern Apennine of Italy

Among the many cases studied of mass-transport deposits in continental margins, the role of basin topography in controlling the types, distribution, architecture and emplacement of such deposits has not been properly remarked. In the western portion of Northern Apennine foothills, masstransport deposits form two composite Messinian mass-wasting bodies that reveal progressive development strictly controlled by basin topography. Extensively analyzed through stratigraphic and structural studies, they form two major elliptical-shaped bodies in map view; maximum 10 kilometres wide, tens of kilometres in length and with estimated volumes of about 250 km3 each, they are elongated parallel to NW-SE oriented thrust fronts. They are coalescing chaotic masses that consist, at the base, of debris flows formed by monogenic gypsum arenite or breccia and decametric blocks of primary gypsum, whereas at the top they are made up of kilometres-wide outliers of pre-gyspum deposits, which slid away from partially preserved headwall scarps. In the external accumulation zone, the mass wasted deposits show imbricate thrust-stacks composed of scraped-off gypsum debris flow deposits. The types, distribution, architecture and emplacement of the studied mass-transport deposits testify the strict control of the wedge-top basins morphology. The internal and steeper flank of the wedge-top basins was representing the depletion zone of sliding masses; whereas, the outer and less steep flank of the wedge-top basins stopped the moving masses and formed the accumulation zones. The relief of the wedge-top basins was progressively modifying during the intra-Messinian tectonic pulse that, affecting the entire Northern Apennine orogenic wedge, triggered the studied mass-transport deposits

Morphology of late Quaternary submarine landslides along the U.S. Atlantic continental margin

The nearly complete coverage of the U.S. Atlantic continental slope and rise by multibeam bathymetry and backscatter imagery provides an opportunity to reevaluate the distribution of submarine landslides along the margin and reassess the controls on their formation. Landslides can be divided into two categories based on their source areas: those sourced in submarine canyons and those sourced on the open continental slope and rise. Landslide distribution is in part controlled by the Quaternary history of the margin. They cover 33% of the continental slope and rise of the glacially influenced New England margin, 16% of the sea floor offshore of the fluvially dominated Middle Atlantic margin, and 13% of the sea floor south of Cape Hatteras. The headwall scarps of open-slope sourced landslides occur mostly on the lower slope and upper rise while they occur mostly on the upper slope in the canyon-sourced ones. The deposits from both landslide categories are generally thin (mostly 20–40 m thick) and comprised primarily of Quaternary material, but the volumes of the open-slope sourced landslide deposits can be larger (1–392 km 3) than the canyon-sourced ones (1–10 km 3). The largest failures are located seaward of shelf-edge deltas along the southern New England margin and near salt domes that breach the sea floor south of Cape Hatteras. The spatial distribution of landslides indicates that earthquakes associated with rebound of the glaciated part of the margin or earthquakes associated with salt domes were probably the primary triggering mechanism although other processes may have pre-conditioned sediments for failure. The largest failures and those that have the potential to generate the largest tsunamis are the open-slope sourced landslides.